In a known process of the this type, as described in U.S. patent application Ser. No. 524,841 assigned to the present assignees (and the corresponding European Patent No. 0044692), initial preparation of the mixture comprises forming a composite slurry in which the waste (in the form of high level radioactive nuclear waste) is mixed with synthetic rock-forming materials. This slurry is fed to a rotary kiln operating at about 750.degree. C. in which the slurry is heated, the substantial quantity of water in the slurry vaporised and the mixture calcined to form a particulate material. The particulate material is an initimate mixture of nuclear waste material and the rock-forming materials. The rotary kiln initiates mineral formation of the synthetic rock and in one embodiment the synthetic rock materials used are those which produce in the final synthetic rock three titanate materials (hollandite BaAl.sub.2 Ti.sub.6 O.sub.16, zirconolite CaZrTi.sub.2 O.sub.7 and perovskite CaTiO.sub.3), plus rutile titanium oxide TiO.sub.2 and a small amount of titanium metal. The titanium metal is for reducing purposes and preferably is added in particulate form and mixed with the calcined powder before the mixture is poured into the container in which subsequently hot uniaxial pressing takes place. Preferably, the container has a bellows-like wall.
The present invention is directed to new and useful alternatives which avoid disadvantages inherent in the use of a wet slurry for mixing the materials initially; it has now been found that a sufficiently intimate mixture of nuclear waste material and synthetic rock-forming materials can be achieved without wet slurry mixing.
It is now pointed out that some of the disadvantages of wet slurry mixing include the following:
(a) To enable the composite slurry to be pumped through the associated equipment, it is necessary for the slurry to have a high water content. Therefore, the process creates, as an undesirable by-product, large amounts of high level radioactive contaminated water which must be treated further to diminish its hazardous nature. PA1 (b) Much larger and more complex handling equipment is required in an active cell. This results in significant expense and much greater capital costs for an active cell, the cost of which is dependent on the volume of the cell. PA1 (c) A lengthy rotary kiln is required since a substantial part of the kiln is devoted to removing water in vapor form from the slurry before the calcining step in the process commences. Capital cost penalties are inherent in this feature. PA1 (d) A relatively expensive off-gas treatment system is required to deal with the considerable volumes of gases emerging from the rotary kiln. PA1 (a) The synthetic rock components are initially formed in an aqueous slurry which is dewatered to form a damp cake of solid particles which is then dried at relatively low temperature (e.g. 130.degree. C.) to form a dry powder. This powder is conditioned if necessary to be in a flowable form for supply to the mixing apparatus. PA1 (b) The powder produced by the process in (a) above may be additionally processed by precompaction to take on a granulated form and this product is then presintered at a low temperature e.g. about 300.degree. C. to improve the mechanical strength of the granules which then provides the feedstock to the mixer. This feedstock has a readily flowable form yet can be produced relatively easily and economically. PA1 (c) The process of (b) above may be further modified to provide the granules in spheridised form to further enhance the flow characteristics of the product. PA1 (d) A slurry of synthetic rock components may be spray dried to form fine particles which are then subjected to a presintering at low temperature e.g. about 300.degree. C. to provide powder feedstock for the mixer. PA1 (e) A highly pourable powder may be formed by a sol-gel process followed by a presintering.
In general terms the present invention is directed at least partially towards avoiding disadvantages now noted in the slurry process and is aimed at providing a simplified process for producing the particulate material for loading into the containers in which the particulate material is to be hot pressed. Although application of the present invention to the production of particulate material for a hot uniaxial pressing is an important application of the present invention, the present invention is not limited in this regard and may equally be applied to the production of material for the formation of synthetic rock incorporating nuclear waste by other processes such as hot isostatic processes.